As an example employing a multilayer piezoelectric element, piezoelectric actuators in which piezoelectric layers and metal layers are alternately stacked one upon another have conventionally been proposed. In general, the piezoelectric actuators can be classified into the following two types, namely simultaneous sintering type, and stacked type in which piezoelectric porcelains consisting of a piezoelectric body and metal layers of plate-like body are alternately stacked one upon another. Among others, the simultaneous sintering type piezoelectric actuators are often used from the viewpoints of lower voltage and manufacturing cost reductions. The simultaneous sintering type piezoelectric actuators facilitate a reduction in layer thickness and have excellent miniaturization and durability.
FIG. 6(a) is a perspective view showing a conventional multilayer piezoelectric element. FIG. 6(b) is a partial perspective view showing the stacked state of piezoelectric layers and metal layers in FIG. 6(a). As shown in FIGS. 6(a) and 6(b), the multilayer piezoelectric element has a stacked body 103, and a pair of external electrodes 105 formed on opposed side surfaces, respectively. The stacked body 103 is configured by alternately stacking piezoelectric layers 101 and metal layers 102. Inactive layers 104 are stacked on both end surfaces of the stacked body 103 in the stacking direction, respectively. The metal layers (internal electrode layers) 102 are not formed entirely over the main surfaces of the piezoelectric layers 101, thereby forming a so-called partial electrode structure. The metal layers 102 in the partial electrode structure are stacked so as to be exposed by every other layer to different side surfaces of the stacked body 103, and the metal layers 102 are connected by every other layer to the pair of external electrodes 105, respectively.
A method of manufacturing the conventional multilayer piezoelectric element is as follows. That is, firstly, a metal paste is printed on a ceramic green sheet containing the raw material of the piezoelectric layers 101, in such a pattern as shown in FIG. 6(b), which forms a predetermined metal layer structure. Then, a plurality of the green sheets with the metal paste printed thereon are stacked one upon another to prepare a stacked forming body. The stacked forming body is then sintered to obtain the stacked body 103. Thereafter, the metal paste is applied to the opposed side surfaces of the stacked body 103, and then sintered to form a pair of the external electrodes 105, resulting in the multilayer piezoelectric element as shown in FIG. 6(a) (for example, refer to Patent Document 1).
As the metal layers 102, in general, an alloy of silver and palladium is often used. In order to simultaneously sinter the piezoelectric layers 101 and the metal layers 102, the metal composition of the metal layers 102 is often set to a 70% by mass of silver and a 30% by mass of palladium (for example, refer to Patent Document 2). The following is the reason that the metal layers 102 composed of the alloy of silver and palladium are used instead of the metal layers consisting only of silver.
That is, the composition of the metal layers 102, which consists only of silver and contains no palladium, causes so-called ion migration phenomenon that when a potential difference is applied to between the opposed metal layers 102, the silver ions in the metal layers 102 migrate through the element surface, from the positive electrode to the negative electrode in the opposed metal layers 102. This phenomenon tends to occur remarkably in the atmosphere of high temperature and high moisture.
On the other hand, for the purpose of forming the metal layers 102 of substantially identical metal filling rate, a metal paste whose metal composition ratio and metal concentration are prepared so as to be substantially the same has conventionally been used. When this metal paste is screen-printed on the ceramic green sheet, the stacked body 103 is prepared by setting a mesh density and a resist thickness to substantially the same condition.
When ceramic green sheets are pressed and stacked, the area where the metal layers 102 are overlapped with each other, and the area where the metal layers 102 are not overlapped with each other have different pressed states, so that the metal layer density might become non-uniform even in the same surface of the metal layer 102. Hence, there has been proposed the method in which the metal filling rate is equalized by forming recess portions in a ceramic tape corresponding to the area where the metal layer 102 should be formed (for example, refer to Patent Document No. 3).
When the abovementioned conventional multilayer piezoelectric element is used as a piezoelectric actuator, it can be driven by connecting and securing lead wires (not shown) by soldering to the external electrodes 105, respectively, and then applying a predetermined potential to between the external electrodes 105. Recently, the multilayer piezoelectric elements are further miniaturized and also required to ensure a large displacement under large pressure. Hence, the abovementioned multilayer piezoelectric element is required to be usable even under severe conditions of higher electric field application and a long-term continuous driving.
In order to meet the abovementioned requirements of the long-term continuous driving under high electric field and high pressure, Patent Document 4 has proposed the element provided with a layer in which the piezoelectric layer 101 has a different thickness. That is, the attempt to relax stress has been made by utilizing the fact that a different thickness causes a different displacement from other layer.
In the stacked type of multilayer piezoelectric element, it has been proposed to control so that the contact resistance of the interface between the metal layer and the piezoelectric layer is high at the center in the stacking direction of the multilayer piezoelectric element, and is lowered toward the both ends, and so that no stress concentrates at the center in the stacking direction of the multilayer piezoelectric element (for example, refer to Patent Document 5).    Patent Document 1: Japanese Unexamined Patent Publication No. 61-133715    Patent Document 2: Japanese Unexamined utility model Publication No. 01-130568    Patent Document 3: Japanese Unexamined Patent Publication No. 10-199750    Patent Document 4: Japanese Unexamined Patent Publication No. 60-86880    Patent Document 5: Japanese Unexamined Patent Publication No. 06-326370